Most gas separation processes do not normally operate at their maximum or nameplate capacity but at some lower capacities due to upstream or downstream conditions. In addition, there is usually a turn-down requirement, i.e., the plant must be capable of running in a range of set points lower than full rate and full rate. Additionally, the plant must also meet other external requirements such as product recovery, product purity, energy requirements, and the like. In most gas process facilities, turn-down requirements are not a problem and can be designed into the plant as the optimum ranges of the equipment are fairly broad and well known and suitable control strategies have been developed. However, in membrane-biased gas processing plants, this is not the case. Membrane units have fairly low turn-down capabilities, i.e., typically from 100% to about 75% of full flow, without sacrificing some other requirement such as product purity. This is due to the fact that as the flow rate decreases, the area per unit flow increases. Since permeation of a gas through a membrane is proportional to the area and the partial pressure difference across the membrane, as the area per unit flow increases, the partial pressure of the non-desired species in the permeated product increases on the feed side forcing more of the gas through the membrane to the permeate side, thereby increasing its concentration on the downstream side. Consequently, as the feed flow to the unit decreases, the product purity also decreases.
Several methods have been developed in an attempt to allow for the turn-down of a membrane system without large sacrifices in product purity. One such method involves removing membrane area by automatically valving off a portion of the membrane modules at predetermined production levels. Such a system which operates by removing membrane area during turn-down of the system is described in U.S. Pat. No. 4,397,661. This patent discloses a permeator system which comprises a plurality of permeator stages each of which receives a feed stream, and the flow of permeate from at least one permeator stage is initiated and terminated at predetermined rates of permeate fluid flow from the permeate system. While this system is said to accomplish turn-down of the unit without sacrificing permeate purity, the system necessarily involves several smaller modular units in parallel, which results in a loss of economy of scale. Additionally, the need to block off membrane area requires additional valves, piping, instrumentation and the like which increases the complexity of the system, the size of the unit and also the capital cost. Step-wise turn-down of these systems also causes discontinuities in performance as a membrane module is valved out, which is generally undesirable in chemical plants because it causes a change in other processing units both upstream and downstream.
A second method which has been used to improve the purity of the permeate stream during turn-down is to raise the permeate pressure. This decreases the flux across the membrane of the lower permeability components faster than the higher permeability; i.e., product, components. By this method product purity is increased, however, such an increase is achieved at the expanse of reduced product recovery. In fact, a significant product recovery decrease is observed as the production rate approaches 80%, and is reduced to the point of being ineffective as the production rate approaches 60%. Consequently, conventional permeate pressure adjustment processes are applicable only as a "fine tuning" control mechanism and are not applicable as a turn-down mechanism, especially as the feed flow decreases as the permeate increases at a fixed purity.